// a small example spin model
    // Peterson's solution to the mutual exclusion problem (1981)
    
    bool turn, flag[2];		// the shared variables, booleans
    byte ncrit;        		// nr of procs in critical section
    
    active [2] proctype user()	// two processes
    {
    	assert(_pid == 0 || _pid == 1);
    again:
    	flag[_pid] = 1;
    	turn = _pid;
    	(flag[1 - _pid] == 0 || turn == 1 - _pid);
    
    	ncrit++;
    	assert(ncrit == 1);	// critical section
    	ncrit--;
    
    	flag[_pid] = 0;
    	goto again
    }
    // analysis: 
    // $ spin -a peterson.pml
    // $ cc -o pan pan.c
    // $ ./pan